Commercial production of Radio Frequency Identification tag (RFID tag) aiming at discriminating IDs by contactless means succeeds one another, and its application is expected in various fields, such as merchandise management, individual attestation, and forgery prevention of banknotes or negotiable securities.
Particularly when the RFID tag is utilized at cash registers in supermarkets, convenience stores etc., it will effectively reduce necessary numbers of staffs and client's waiting time at cash registers, since checkout can be instantly performed without the need for taking out goods one by one, thereby improving operational efficiency and quality of customer service.
Discernment of IDs is performed when a transponder responds its ID to a query from an interrogator, thus it is necessary to query each transponder individually. However, when multiple transponders are in the same communication area, their responses collide making it impossible to discriminate their IDs. Therefore, response requirements are specified in the queries so as to make only the transponders meeting the requirements make responses to the queries. Since there is no transponder with the same ID, the most efficient method of preventing a collision by way of setting the response requirements is to utilize IDs of the transponders themselves as response requirements.
In order to discriminate IDs by this method, the interrogator needs to query every one of existing transponders to make them respond their IDs. However, it is not practical to use this method in a place like a supermarket, where a vast quantity of goods is dealt with, because it is extremely time consuming to query IDs of all the goods in the supermarket every time a customer checkout their purchases at a cash register.
In order to solve this problem, various multi-reading methods have been proposed, but in these conventional methods, an interrogator queries the symbols 1/0 in each of the binary digits (bits) which constitute ID in order, and when its response disagrees with the query, further responses from the transponder having that ID is forbidden. The interrogator reads out IDs of the transponders that remain until the last one by one as detection numbers. For this reason, the conventional methods are problematic in that more queries become necessary as the bit length becomes long, requiring a long time to read one ID and even much longer time to read numbers of IDs of all the transponders.
Further disadvantageously, a write-in processing will be necessary on the transponders with the conventional methods in order to forbid unwanted responses, and this requires additional costs for necessary parts and overhead for the write-in processing.
One object of the present invention is therefore to provide a method for multi-reading a plurality of IDs, which can efficiently discriminate IDs in a short time even if the bit length is long, and which does not need the write-in processing on the transponders.